Is it possible to make half of an object semi transparent and the other half non-transparent?
Yes, this can be done by assigning different materials to the same object as shown below.


I added a cube to the scene at origin and scaled it to be bigger
Then I added a sphere to the origin which is now located inside the cube
I added a material to the cube as shown below:





I then checked and turned on the transparency and decreased the the Alpha value as shown below:





Then I added a new material and set its color to red as shown below:





Then I switched to edit mode, switched to face mode and selected couple of faces and clicked assigned to the red material as shown below:





Then render and as shown below the grey material is transparent showing the sphere inside while the red material is not: